This invention relates to mold structures and methods for holding a sheet of material during molding and, more particularly, to mold structures and methods for holding a sheet of material during molding of a gasket or casing on the sheet of material, such as glass, to form modular window assemblies for vehicles, architectural applications, and the like.
In recent years the manufacturing of automobiles and other vehicles has included the use of light weight, strong modular window assemblies which are made and installed as a single unit from the outside of a vehicle. Much less labor is required for such installation than has been previously known. Examples of this type of modular window assembly are shown in U.S. Pat. Nos. 4,072,340 and 4,139,234. Generally, these assemblies include a sheet of glass and a peripheral casing or gasket of molded polymeric material which extends around at least portions of the periphery of the glass. In some cases, an integral frame is embedded within the casing or gasket. In other cases, an outer frame or bezel is secured to the gasket or casing with integral portions of the bezel molded into the casing. In yet other cases, the casing or gasket is simply molded from material such as polyvinyl chloride (PVC) or reaction injection molded polyurethane and the entire assembly is secured to the vehicle by strong adhesives and/or fasteners molded into and protruding from the gasket and/or attached to the glass itself.
The production of such modular window assemblies requires the combination of a sheet of glass and a polymeric gasket. The glass is configured to cover the shape of an opening in a vehicle, such as a windshield opening, a back light opening, or a side view opening as well as being curved to the contour of the overall vehicle body shape. The gasket or casing must be formed and adhered to a portion of the glass such as its periphery to not only properly seal the window around the vehicle body opening but also to provide appropriate trim or body filling portions to meet required appearance standards. The gasket or casing is typically applied to the periphery or another portion of the glass sheet by reaction injection molding or PVC molding.
In reaction injection molding (RIM), two agents are injected into the mold cavity of a molding apparatus. The agents combine in a chemical reaction to form a thermosetting, resinous urethane material. Since RIM is a chemical reaction molded process, the pressure required for the process is low- on the order of 50 psi.
In PVC molding, the PVC is melted and then injected into the mold cavity of the mold assembly. The molten PVC is typically injected under pressures ranging from 2,000 psi to 5,000 psi. The cycle time of the PVC molding process is much faster that RIM and entails lower material costs than reaction injection molding, as the cost of PVC is lower than the cost of the agents required in RIM. Moreover unlike RIM, the PVC is thermoplastic and recyclable and, therefore, scraps from the molding process may be reheated and reused again in another cycle, thereby helping to reduce overall material costs.
One significant problem frequently encountered in both such methods of manufacture is the breakage of glass when the mold sections used to make such assemblies are closed. Glass sheets which are curved, shaped, and contoured include a relatively wide range of permissible piece to piece variation in their contour. Consequently, when rigid mold sections are closed around a sheet having a greater variation in contour than the mold permits, such sheet may be bent beyond its fracture point or be broken, chipped, damaged, or otherwise made unsuitable for use. This is especially true for more highly contoured glass sheets in recently developed aerodynamically styled vehicles in the United States and elsewhere. In PVC molding, the percentage of glass breakage is typically greater than for RIM. In conventional PVC molding, the shut off surfaces for the PVC, which close the mold cavity along and around the glass sheet, comprise rigid steel surfaces, which are conventionally required in order to resist the high pressures associated with the PVC molding process. Because these surfaces are rigid, they cannot adequately accommodate the degree of tolerance variations exhibited in highly contoured windows. Moreover, with the high pressures associated with the PVC molding process, the holding force of the mold sections is significantly greater than for RIM. The combined effect is to have a greater percentage of glass breakage in PVC molding than in RIM.
A second major problem encountered in the manufacture of such modular window assemblies, is the overflow or flashing of polymeric or other flowable molding material out of the desired mold cavity at the peripheral edge of the glass sheet either onto the viewing areas of the glass or away from the gasket or casing position. Overflow or flashing can be reduced by increasing the holding pressure on the sheet. However, this will have the adverse effect of increasing the percentage of breakage. This unsightly "flash" or mold overflow requires manual trimming with knives or razor blades after molding. Extensive trimming raises the cost of such assemblies or requires scrapping such parts if the overflow or flash is extensive.
In reaction injection molding (RIM), resilient seals are often provided to stop the flash of the sealant material. These seals are typically located at the distal edge of the mold assemblies and abut the mold cavity provided for the seal material. Heretofore, these seals have been unavailable for use in PVC molding, because the seals cannot withstand the high pressure associated with the PVC molding process. If use was attempted in PVC molding, the seals typically ruptured or released an uncontrolled amount of gasket material onto the viewing the surface of the glass. In the extreme case, the seals are blown out and the glass sheet breaks.
Accordingly, there is a need for a high pressure mold assembly and method for holding a sheet of glass during a molding process with PVC, other thermoplastic or resinous materials, or other materials molded under high pressure that will accommodate sheets of glass with a greater range of dimensional variations and will significantly reduce the breakage of the sheets during the molding process, while maintaining or controlling the amount of flash or overflow of the flowable PVC or other moldable material onto the sheet of glass.